1. Field of the Invention
The invention relates to a method and an apparatus for avoiding or reducing trouble-induced time delays in solvent recovery from production processes.
2. Description of the Prior Art
In many industrial processes, for example coating and drying of tapes, foils, etc., such as audio or video tapes, solvent vapours arise which must be collected and recovered because they are mostly explosive when mixed with air and almost always harmful to health.
From DE-A-30 42 082 a method is known for recovering solvents in which the exit air containing solvent vapours, for example from such a drier, is conducted via adsorbent charges. The exit air purified by adsorption of the solvents is thereafter released into the atmosphere. A second circuit with inert gas serves for the desorption of the solvent-charged adsorbent charges.
DE-A-39 07 437 discloses a method for removing solvent constituents from the exit air of painting or enamelling plants and the like in which the exit air from such a painting plant is divided into two subflows. One subflow is returned directly to the plant after reheating whilst the other subflow is returned to the plant only after purifying.
For reliable operation of these plants, due to the use of air as flushing gas, particular attention must be paid to ensuring that the concentration of the solvent vapours in such a plant remains so low that the ignition or explosion limit of the mixture of air and solvent is not reached. As a result, large amounts of air must be conducted through the plant and after extraction from the plant freed from the entrained solvent vapours. Moreover, moisture is entrained with the air supplied into the plant and is frequently present in a greater amount than the actual solvent. This leads to difficulties in all exit air purifying methods of this type and additional apparatus and energy expenditure.
A further method of purifying a carrier gas by adsorption of the solvent vapours contained therein is known from DE-C-38 10 705. In this case as well the purified exit gas is released to the environment.
However, to enable the demands of high operational reliability and economical use of energy to be met, the driers, and frequently also the coaters, are encapsulated, shielded from the access of air and held under an inert atmosphere, for example nitrogen. At the same time, the locks are supplied with inert gas.
Corresponding plants are described in the technical journal "cav", number 1988/9, on pages 30, 32, 34 and 39, and in the technical journal "Technische Rundschau", number 23/87 on pages 46 and 47. In the method which was disclosed in the "Technische Rundschau" for "solvent recycling on inert gas basis" the mixture of solvent and inert gas extracted from the plant is led in a primary circuit via a socalled recovery with recuperator and cooler and the inert gas freed from the solvent constituents returned to the plant again. Via a further circuit, the socalled secondary circuit, the locks through which the plant is encapsulated from the ambient air are also supplied with purified inert gas.
A modern solvent recovery plant as apparent for example from elder European patent application 90 116 885.6 thus even has two circuits partially separate from each other, i.e.
a carrier gas circuit from which the solvent condenses and the purified carrier gas is returned to the production plant again, and PA0 a low-temperature section with which the locks of the manufacturing plant, for example a drier, are supplied.
In continuous production undesirable time delays occur whenever the operation of the drier must be interrupted, for example because of tearing of the tape, in order to open the plant to carry out repair measures. For safety technical reasons, this can only be done when the solvent content in the drier atmosphere has been reduced by the solvent recovery apparatus to such an extent that there is no longer any danger of explosion even on mixing with the ambient air.
The time necessary to do this is governed substantially by the gas throughflow capacity and thus the refrigeration power of the low-temperature section. The low-temperature section is however only designed for its original purpose of supplying the locks with the carrier gas.